Magnetic recording media such as video tapes, audio tapes, and computer tapes generally comprise a non-magnetic support having provided thereon a magnetic layer comprising a binder having dispersed therein a ferromagnetic powder, e.g., a ferromagnetic iron oxide powder or a ferromagnetic alloy powder.
With the latest developments in high-density and long-play magnetic recording media, improvement in electromagnetic characteristics has been desired to meet ever increasing demands for higher image and/or sound quality. High-density recording requires magnetic recording media which provide achievement of high output and low noise, as well as improved reproducing (i.e., play back) and recording hardware such as a VTR. Possible approaches to high output include use of a ferromagnetic alloy powder having a high coercive force and a high saturation magnetization (.sigma.s), use of a ferromagnetic powder having a reduced size to increase packing density of the magnetic layer, and improvement of the surface properties of the magnetic layer to minimize spacing loss on contact with a magnetic head. Possible approaches to long-play include reduction of thickness of the magnetic layer or of the non-magnetic support.
Past proposals for improving image or sound quality include size reduction of a Co-containing iron oxide powder, use of a ferromagnetic alloy fine powder, and use of a multi-layered magnetic layer in which an upper magnetic layer contains a ferromagnetic alloy powder and a lower magnetic layer contains a ferromagnetic iron oxide fine powder to provide improved electromagnetic characteristics over a broad frequency range. Each of these techniques require uniform dispersion of the fine particles of a ferromagnetic powder. To meet this requirement, various studies have been conducted relating to the binder for use in the magnetic layer. For example, the binder proposed in JP-A-59-5424 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") contains a resin having at least one polar group selected from --SO.sub.3 M, --OSO.sub.3 M, --COOM, and --OPO.sub.3 M.sub.2, wherein M represents a hydrogen atom, an alkali metal, or an ammonium base (the plural M's may be the same or different), to improve dispersibility or the degree of packing of a ferromagnetic powder. On the other hand, size reduction of a ferromagnetic powder or improvement in surface properties of a magnetic layer tends to deteriorate running durability, such as clogging of a magnetic head. Consequently, it has been proposed to harden the magnetic layer by crosslinking the polar group with an isocyanate compound or by incorporating therein polyurethane having a high urethane bond concentration.
On the other hand, while polyethylene terephthalate is widely employed as a non-magnetic support for these magnetic recording media, the use of polyethylene naphthalate, polyamide, etc. has recently been extended as a thin and hard base suitable for long playing. Any one of these resin films is stretched to have high crystallinity, and thereby provides excellent organic solvent resistance and mechanical strength.
A magnetic layer obtained by coating a magnetic coating composition containing the above-described binder on a non-magnetic support has a high modulus of elasticity due to the high packing density of a ferromagnetic powder. However, since the magnetic layer has numerous microvoids unlike composite materials in general, it has a small elongation at break and has insufficient mechanical strength against large deformation. Therefore, a magnetic layer coated directly on a non-magnetic support without an undercoating layer therebetween is apt to be broken upon application of force. Besides, a magnetic layer with microvoids has poor adhesion to a non-magnetic support. The easiest and most effective solution to these problems is to provide an undercoating layer typically employed with general coatings.
With respect to an undercoating layer, compositions for an undercoating layer are disclosed in JP-B-47-22071 (the term "JP-B" as used herein means an "examined Japanese patent publication") JP-B-49-10243, JP-A-52-42703, and JP-A-59-19230. In particular, undercoating layer compositions containing a polyester resin considered to have excellent adhesion to polyethylene terephthalate are disclosed in JP-B-62-37451, JP-A-60-11358, JP-A-60-19522, JP-A-60-21250, and JP-A-61-2654510. Furthermore, JP-A-1-245421 discloses a specific structure of a polyester resin for use in an undercoating layer. However, none of the known undercoating layers exhibits satisfactory adhesion in the case where a magnetic layer coated thereon having an increased modulus of elasticity and increased hardness for improvement of running durability, or an increased packing density of the ferromagnetic powder and thereby increased hardness for improvement of electromagnetic characteristics is used, or in the case where a hard non-magnetic support, such as polyethylene naphthalate or polyamide is used.